1. Field of the Invention
This invention relates to a method for producing a semiconductor substrate that can be applied in semiconductor devices or liquid-crystal display devices, and more particularly relates to a method for depositing Si film by sputtering. It also relates to a semiconductor substrate, and a semiconductor device or liquid-crystal display device having applied it.
2. Related Background Art
Semiconductor devices used in integrated circuits are commonly comprised of a thin-film multi-layered product formed by depositing a number of thin films layer by layer, and the quality of each thin film and the state of interfaces between thin films have a great influence on the performance of devices. Hence, high-level thin-film forming and multilayer forming techniques are indispensable for providing high-performance semiconductor devices. In particular, what is called the epitaxial growth technique, which forms an additional high-quality crystal on a crystal face, is a thin-film forming technique indispensable for existing semiconductor techniques, and much research and development is being done on these techniques since they greatly influencing device performance. A thin film obtained by epitaxial growth is called an epitaxial film.
In conventional epitaxial growth techniques, CVD processes have been prevalent. However, film formation by CVD is, in general, carried out by a high-temperature process. For example, a film formation process for a silicon film (Si film) is carried out at a high temperature of 1,000.degree. C. or above. This has brought about problems such as limitations on processes which can be used and a high production cost, ascribable to the high temperature. Moreover, it has now become difficult to meet demand for making dopant profiles shallower or sharper, which is a recent demand attributable to devices having been more highly integrated and having been made to have a higher performance.
Accordingly, as processes that can meet such demands, low-temperature epitaxial growth processes have been recently reported, including MBE (molecular beam epitaxy; A. Ishikawa and Y. Shiraishi, J. Electrochem. Soc., 133, 666, 1986), as well as FOCVD that carries out film formation by mixing a gaseous material and a halogen type oxidizing agent to cause chemical reaction so that a precursor serving as a feeding source of a film forming material is formed (U.S. Pat. No. 4,800,173), HRCVD that carries out film formation by separately introducing into a film forming space, gaseous materials activated in different activation spaces (U.S. Pat. No. 4,835,005), PIVD (partly ionized vapor deposition) that utilizes an ion beam process (T. Itoh, T. Nakamura, M. Muromachi and T. Sugiyama, Jpn. J. Appl. Phys. 16553, 1977), IBE (ion beam epitaxy; P. C. Zalm and J. Beckers, Appl. Phys. Lett. 41, 167, 1982), and ICBD (ion cluster beam deposition; I. Yamada, F. W. Saris, T. Takagi, K. Matsubara, H. Takaoka and S. Ishiyama, Jpn. J. Appl. Phys. 19, L181, 1980).
However, MBE requires a 800.degree. C. or higher temperature process to form a high-quality epitaxial thin film and high-density doping is difficult. FOCVD and HRCVD, which utilize chemical reactions, have the problem that by-products may be formed and be incorporated as impurities into films. In what is called the ion beam process such as IBE or ICBD, the ions used have so much energy that substrates may be damaged, and no high-quality thin films usable in semiconductor devices have been obtained under existing circumstances.
The present inventors have reported Si epitaxial growth carried out by RF-DC combined bias sputtering, employing a method in which the surface layer is activated while controlling any damage on the substrate by precise control of ion energy (T. Ohmi, T. Ichikawa, et al., J. Appl. Phys. Vol.66, p.4756, 1989).
Hitherto, sputtering has not been considered utilizable to form epitaxial films because of difficulty in the controlling the damage of substrates due to ion energy. It, however, has many other advantages, i.e.;
(1) it can readily form a large-area film; PA1 (2) it can achieve a relatively simple device construction; PA1 (3) it can match usual semiconductor processes; and PA1 (4) it allows easy control of reaction systems. PA1 (1) When processing gas, argon atoms are incorporated into an epitaxial film in a concentration of 8.times.10.sup.18 cm.sup.-3 or more, the resulting film may have poor quality, and the quality may become extremely poor in a device fabrication process carried out at temperatures higher than film formation temperatures. PA1 (2) The carbon can not be completely prevented from being included in films, and the same problem as in paragraph (1) may arise when carbon is included in a concentration of 1.times.10.sup.18 cm.sup.-3 or more. PA1 (3) The process has a poor step coverage. PA1 (4) An epitaxial film formed on the (111) crystal face of an Si substrate or a heteroepitaxial film such as an SiGe film formed on an Si substrate may have defects therein chiefly caused by many film deposition defects, to make the state of their interface unsatisfactory. PA1 (1) Argon atoms deposit on a substrate at a large coefficient when the substrate has a low-temperature of about 300.degree. C. PA1 (2) The energy possessed by atoms such as Si that undergo epitaxial growth is so small that their surface migration may become insufficient. PA1 (3) Carbon atoms are in the state they tend to be attached to the activated substrate surface.
The foregoing RF-DC combined bias sputtering can keep the advantages of sputtering in the epitaxial film forming techniques.
The RF-DC combined bias sputtering having such advantages, however, has the following problems.
Thus, according to such epitaxial growth, it is very difficult to grow a monocrystal on an amorphous substrate or to grow a crystal having a lattice constant and a coefficient of thermal expansion that are different from those of the substrate. This has imposed restrictions on substrate materials usable and the types of films grown. Meanwhile, in research and development in recent years, development is energetically being made on three-dimensional integrated circuits that aim at higher integration and higher multifunction by forming semiconductor devices on a substrate layer by layer, and also on solar cells, switching transistors of liquid-crystal picture elements comprised of devices arranged in array arrays, etc., which are formed by depositing semiconductor materials on inexpensive glass substrates. Accordingly, it has become important to provide techniques by which high-quality semiconductor thin-film layers can be formed on an amorphous substrate having a structure that is common to these devices. In recent years, TFT thin-film forming techniques for achieving such a structure are remarkably improved. For example, ion implantation is carried out on a polycrystalline semiconductor thin film or amorphous semiconductor thin film on an amorphous substrate to make the film completely amorphous, followed by heat treatment to obtain a polycrystalline semiconductor thin film having a grain diameter as large as several .mu.m (T. Noguchi et al., J. Electrochem. Soc. Vol. 134, No. 7, p.1771, 1987), or an amorphous semiconductor thin film deposited on an amorphous substrate by sputtering is exposed to laser light to obtain a polycrystalline semiconductor thin film having a grain diameter of about 400 .ANG. (1989 Spring Season Applied Physics Society, 3P-ZH-15, 16). In both examples, MOS devices having fairly good electrical characteristics and showing a high mobility are fabricated.
Such an ion implantation method and a laser melting method, however, both essentially require a high-temperature process, and hence they not only are difficult to apply to three-dimensional integrated circuits, but also hinder the large-area uniform thin-film formation, low-temperature process and simple process that are recently required for the purpose of adaptation to large-area substrates and cost reduction. Accordingly, it is sought to provide a high-quality thin-film forming technique by which high-quality devices can be formed on amorphous substrates by a simple low-temperature process that employs neither ion implantation nor laser melting. Meanwhile sputtering and glow discharge, are processes for forming semiconductor thin films on low-temperature large-area amorphous substrates without use of the ion implantation or laser melting. Under existing circumstances, however, in view of electrical characteristics such as mobility, no satisfactory device characteristics can be obtained in the semiconductor thin films deposited by these processes, compared with the above processes.